1. Field of the Invention
This invention relates generally to color video cameras and, more particularly, is directed to the auto-white balance control of such cameras.
2. DESCRIPTION OF THE PRIOR ART
Color video cameras require adjustment of the white balance thereof in dependence on the color temperature of the source of light illuminating a scene in the field of view of the camera. Manual white-balance adjustment has been conventionally performed by placing a white object in the field of view of the camera, for example, by mounting a white cap on the lens, and then setting the gains of the several color signals so as to establish a predetermined ratio among such color signals. However, manual white-balance adjustment requires readjustment of the white-balance whenever the light source changes, and thus is bothersome. On the other hand, a color video camera having a full automatic white-balance control system can automatically adjust the gains of the several color signals whenever the color of the light source is changed without any intervention by the camera operator. However, the conventional fully automatic white-balance control system may not achieve precise white-balance adjustment, and thus is not reliable in operation. More specifically, the conventional fully automatic white-balance control system operates on the premise that all colors are uniformly distributed in the picture or scene in the field of view of a camera so that summing all of the colors in the picture should result in white. Based on such premise, the conventional fully automatic white-balance control system sets the gains of the several color signals with reference to the sum or integral of the color signals for the entire picture which is presumed to be white. However, the several colors are not distributed uniformly in the picture so that the sum or integral of the color signals does not substantially correspond to white unless the picture itself contains a fairly large white area. Therefore, the conventional fully automatic white balance control system cannot attain accurate white-balance adjustment for all pictures or scenes in the field of view of the camera.
By reason of the foregoing, a color video camera having a fully automatic white-balance control system, as aforesaid, has been further provided with a manual white-balance adjustment for use in achieving accurate white-balance adjustment in those cases where, by reason of the nature of the scene or picture in the field of view of the camera, accurate white-balance adjustment by the fully automatic system is not to be expected. In such case, of course, the manual white-balance adjustment requires the placement of a white cap on the lens, as previously described, so that the use of the manual white-balance adjustment is inconvenient.